Put some sticky strip or glue dots in the corners of the wrong side of the plastic sheet with the printed images on it. In other words, the print will be right side up when viewed from the back of the case. Then slide the sheet in between the case and its outside sleeve as shown below. Push against your tape/glue dots to make sure they stick.
Now you place your stamps directly to the inside of the case as you usually do with clear mount stamps.
I have now done this to all of my photopolymer stamp sets. I won't have to mess with two sticky sheets and the icky mess of the print sticking to my stamps. This is much more convenient to use too:)